Resolves #1111 and extract url generation method

This pull request resolves "Issue #1111 - Improve message errors showing the name of the file that contains errors" on Post#render
This may be very helpfull when we have a problem rendering an specific post, until previously we can't know which post was the problem

In addition, I extracted a new method Post#generate_url to generate an url for the post and call this generation from Post#url